I'd say physical coordination is the most important trait. No matter how good a trainer he has, a physically uncoordinated fighter won't get far. Out of the traits you listed I'd say conditioning is the most important, but I've seen fighters be successful even without that.
For a long time I thought speed was the most important trait, but then I remembered fighters like George Foreman who is slow as a sloth. That fight against Moorer he was getting his butt kicked badly, but all he need was that 1 punch. While not slow, Duran, Chavez sr., Holyfield, Lewis, Bowe, Vitali, Cotto weren't/aren't particularly fast either. Many of these guys had reach, however, which is basically the equivalent of speed.
In general, it's better to be quite good in all departments than brilliant in one and crap in all the others.
What makes boxing great is the different strengths fighters bring to the table.